This invention relates to educational demonstrations, and more specifically to the methods and apparatus used to demonstrate the principles of activation energy, the SN2 reaction, and the potential energy of a covalent bond as a function of internuclear distance.
A brief description of activation energy and the SN2 reaction will be discussed followed by a description of the potential energy of a covalent bond as a function of internuclear distance. Even if a chemical reaction is energetically favorable for a reaction to take place, considerable experimental evidence exists showing that if covalent bonds are broken in this reaction, the reactants must go up an energy hill first before the reaction can take place. This phenomenon is referred to as activation energy represented by FIG. 1. In other words energy must be put into the system before a chemical reaction can take place. Any reaction in which bonds are broken will have energy of activation greater than zero. Every thing else held constant, a low energy of activation means a reaction will take place rapidly; a high energy of activation means that a reaction will take place slowly.
An SNN2 reaction is a bimolecular Nucleophilic substitution. The term Bimolecular refers to the observation that two species are involved in the rate-determining step. For example when the concentration of one of the species involved in this type of reaction is doubled the rate of the reaction is also doubled. When the concentration of both species are doubled the rate of the reaction increases by four times. The rate of this reaction is said to be second order overall, hence the name SN2 reaction. The mechanism for this reaction was one based on the ideas proposed by Edward Hughes and Christopher Ingold in 1937 illustrated in FIG. 3. According to this mechanism a nucleophile attacks the carbon bearing the leaving group from the backside. The orbital containing the electron pair of the nucleophile begins to bond with the carbon atom containing the leaving group; consequently, the bond between the carbon atom and the leaving group weakens. As this happens, the carbon atom has its configuration turned inside out, it becomes inverted and the leaving group is pushed away.
Covalent bonds are central to the study of chemistry. FIG. 2 illustrates what happens to the total energy of a system when a covalent bond is formed. For example when two hydrogen atoms combine to form a hydrogen molecule and their electrons with opposite spins are brought closer and closer together. When the atoms are far apart their total energy is that of two isolated hydrogen atoms. As the atoms move closer together each nucleus increasingly attracts the other""s electron resulting in a lowering of the energy of the total system; thus the two atoms attract each other. This attraction more than compensates for the repulsive force between the two nuclei. When the two hydrogen atoms finally bond the equilibrium bond length for the hydrogen molecule is formed and the energy of the total system is at its lowest. In other words the most stable energy state is obtained. If the nuclei are moved closer together the repulsion of the two positively charged nuclei predominates, and the energy of the system rises.
This invention is intended to be used as an Educational demonstration for college level instruction. The invention allows students to visualize the phenomenon of activation energy. Rearrange the unit to demonstrate the SN2 reaction and the potential energy of a covalent bond as a function of internuclear distance. The demonstration can be used as a visual aid to enhance the students understanding of the material presented during the instructors lecture time, or the demonstration can be used directly by the student as a laboratory exercise. The student will gain important graphing skills by using this invention to plot the activation energy curve. The device is made with the most powerful annular magnets commercially available. The size and shape of the annular magnets produces the unusual effects that characterize this invention. An acrylic rod, containing a metric height scale for easy measurement, aligns the magnets.
Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.